1. Field of the Invention
The present invention relates to a distance relay apparatus that protects a power system and, more particularly, to a distance relay apparatus that detects a close-up fault of a power system at high speed.
2. Description of the Related Art
A distance relay apparatus includes a directional relay element for determining the direction of the point of a fault viewed from the location of a relay element and a distance relay element for obtaining the distance from the location of the relay element to the point of the fault. By combining the directional relay element and the distance relay element with each other, the distance relay apparatus determines whether the fault occurs within a protective zone. Then, the distance relay apparatus outputs a circuit breaker trip signal immediately after it determines that the fault occurs within the protective zone.
FIG. 25 is a diagram of operating characteristics of a mho relay element that serves as a directional relay element and a reactance relay element that serves as a distance relay element. The characteristics are represented as an impedance plane. In FIG. 25, the overlapping portion of the characteristics of both the relays, or the hatched portion is a zone-1 protective region serving as a distance relay apparatus.
FIG. 26 shows a logic sequence circuit that outputs a zone-1 operating signal of the distance relay apparatus including both the mho relay element and the reactance relay element.
In FIG. 26, the mho relay element is denoted as Mho and the reactance relay element is denoted as X1. Reference numeral 11 indicates an AND circuit that operates when both of these relays operate (output an operating signal “1”). The AND circuit is so configured that it outputs a zone-1 operating signal Z1 of the distance relay apparatus.
The protective zone of the reactance relay element X1 is generally set at about 80% of the entire length of a power transmission line from a terminal at one end to a terminal at the remote end. If the distance relay apparatus is used as a main protective apparatus, it needs to operate at high speed when a system fault occurs within the zone-1 operating zone of 80%. When a system fault occurs in the zone-2 operating zone of 20% that corresponds to the protective zone, the operating time of the distance relay apparatus is delayed by a timer such that the apparatus operates for backup protection. FIG. 27 shows zone-1 and zone-2 operating zones of distance relay apparatuses Ry-A and Ry-B that are provided at opposing A and B terminals, respectively.
When the distance relay element computes the distance from the location of the relay element to the point of a fault shorter than the actual distance because of an error due to the characteristics of an input transformer and an error in computation in a computing section, it trips even for a fault that occurs outside the original protective zone. This is called an overreach. The overreach may have a great influence on a power system. The distance relay element is therefore required to compute the distance to the point of a fault with high precision in order to prevent an overreach from occurring.
The directional relay element is also required to perform the same high-precision computation as the distance relay element because the directional relay element will trip though a reverse fault occurs if it makes an error in computation.
FIG. 28 is a block diagram of a distance relay apparatus that is configured by a digital protective relay. A power transformer PT and a current transformer CT transform voltage V and current I of a power system TL, respectively and auxiliary transformers 1-1 and 1-2 transform the voltage and current into ones each having a given level. The voltage and current are input to an analog filter 2 from the auxiliary transformers 1-1 and 1-2 and high-frequency noise is eliminated therefrom. The output of the analog filter 2 is supplied to a sample-and-hold circuit 3 and sampled at regular sampling intervals. A multiplexer 4 permutes the sampling outputs of the sample-and-hold circuit 3 in time series and an A/D converter 5 converts them into digital data. The digital data is input to a digital filter 67. The digital filter 67 serves to eliminate components that have an adverse influence on protective relay computation, e.g., DC components. Since the digital filter 67 is described in IEEJ Lecture on Protection Relay, p. 110, Table 6.1, its detail descriptions are omitted. A computing circuit 89 receives an output of the digital filter 67 and performs computations on the direction of a fault and the measurement of distance. The results of the computations are processed on the basis of given logic and output as an instruction of the relay element.
As a transfer function of the digital filter 67, the time length of data for use is increased and, in other words, a number of items of sampling data are used to make a higher-performance filter. On the other hand, a long time is required for filtering and a response is delayed.
To compute a current level in the computing circuit 89, there are a plurality of algorithms from a relay computation algorithm using a large number of items of sampling data to that using a smaller number of items of sampling data. Such a relay computation algorithm is described in, for example, IEEJ Technical Report No. 641, “Basic Technology of Protective Relay System,” p. 85. If the computing circuit 89 uses a number of items of data by lengthening the data window of data (time length of data for use) like the digital filter 67, the computation precision is improved but the response is generally delayed.
As described above, it is necessary to select one resistant to noise, as a filter or a relay computing system in order to improve the precision of distance measurement, whereas operating time will be lengthened.
On the other hand, when a fault (close-up fault) occurs at a point close to the bus of a power system, a fault current is large and has a great influence on the power system. It is thus expected that a relay element will be operated at high speed. A conventional protective relay employs a number of data items in order to improve the precision of distance measurement for a fault that has occurred near the boundary of a protective zone (a fault that has occurred at a distance of 80% from a terminal at one end as described above). Consequently, even in a close-up fault that is likely to have an influence on the system, the operating time of the protective relay cannot be shortened.